Cutting is used in the manufacture of ophthalmic lenses, particularly in producing customized progressive lenses which have aspheric surfaces and cannot be made by traditional optical polishing. There are, however, significant drawbacks to cutting lenses, primarily related to difficulties in controlling the surface shape to the required optical accuracy and to the quality of the surface finish.
To understand these drawbacks, it is useful to consider in some detail a conventional cutting lathe, and the spiral cutting action it uses to produce a surface geometry. A typical three-axis lathe is shown schematically in FIG. 1. A work-piece 10 is attached to a spindle 12 by a chuck 14 and rotated about the spindle axis 16 (also known as the “C” axis). The surface of work-piece 10 is shaped by moving a cutting tool 18 in toward axis 16 (along the “X-axis” direction), while adjusting its position in the “Z-axis” direction, parallel to the spindle axis 16. A Computer Numerical Controlled (“CNC”) three-axis lathe may have traditional single point cutting tools, or multi-fluted, high-speed rotating cutters, similar to a milling machine. In both, the surface geometry is cut by the tool following a spiral cutting path, and the last piece of material to be cut is at the very center of the rotating work piece 10.
The lathe spindle 12 is typically rotated at a constant speed. This results in the surface-cutting-speed decreasing as the cutting tool 18 moves in toward the center of the work piece 10, with a corresponding change in the cutting force between the tool and the lens being formed. This change in cutting force introduces a gradual error into the shape of the surface being formed. Even if the lathe has a continuously variable spindle speed, it has an upper limit to that speed, and, therefore, a radius beyond which it is not possible to maintain a constant surface-cutting-speed. At that radius, the cutting force will change and the error will begin to be introduced.
Another problem is a characteristic center defect that occurs if the lens design requires removing material all the way to the center of the work piece 10. This center defect is a small depression, generated because the cutting force drops suddenly to zero when the final piece of material is removed. The cutting force bends the tip of the tool slightly away from the lens surface during cutting. When the force drops suddenly to zero, the cutting tool holder relaxes, and the tool moves in toward the still rotating lens, scooping out a small dimple at the lens center. Even high precision lathes produce center defects on the order of 1 to 5 microns deep, creating blemishes that are often visible and cosmetically undesirable.
In prism shaped lens designs in which the central region is flat, the sudden loss of cutting force occurs at a larger radius, but still has a noticeable effect in the form of a bump on the uphill side of the prism and a hole on the downhill side. The bump is typically about 2 microns high, and the hole is typically about 2 microns deep.
Because these loss-of-force defects are a complex function of tool sharpness, surface geometry, material properties and machine characteristics, they are essentially unpredictable. Attempts to compensate for them using software algorithms often produce worse defects.
Another problem in cutting lenses using a lathe stems from the quality of a cut surface being a function of cutting speed. For optimum efficiency, surface speed should be adjusted to produce the highest quality finish only at the lens radii where it is required. In traditional lathes, cutting speed is a fixed function of radius, and cannot be varied.
A further problem with lathes is that imperfections in the shape of the cutting tool (also know as “form imperfections”) are transferred to the lens surface. A typical high-quality cutting tool used in a lathe is a single-point diamond chip, ground to a radius of about 2 mm. The accuracy of the edge of such a diamond is, however, only about 2 microns. This inaccuracy takes the form of scalloping (also known as “waviness”) and is transferred directly on to the lens, i.e., a waviness defect of 2 microns on a cutting tool becomes a lens surface waviness defect of 2 microns. Waviness defects are completely unpredictable and cannot be compensated for by software. Controlled Waviness Tools are available at considerable cost. They also wear or chip quickly to a point where they are outside of specification, so their advantage is costly and short lived.
These cutting defects are traditionally overcome by polishing the cut lens, but this introduces a further deviation from optimal lens shape (also known as a “form error” in the lens surface). In practice, a compromise has to be made between the amount of form error introduced and the amount of the visible, cosmetic center defect that is removed.
Another traditional form of machining surfaces is milling. A typical milling machine is shown schematically in FIG. 2. FIG. 2 a shows a side elevation in which a work piece 10 is held in a milling chuck 20 and moved in a raster pattern in the X-Y plane, while a rotating cutting tool 22 is moved up and down along the Z-axis. FIG. 2B is a plan view showing the raster pattern 24 that is the effective path of the cutting tool relative to the work piece 10. Although traditional milling allows independent control of surface cutting speed and position, and has no problems due to the cutting force suddenly dropping to zero, it produces very low quality surface finishes. FIG. 2C illustrates, in a magnified cross-section, that a typical mill cut surface 26 will frequently have a highly scalloped finish. The scalloping (also known as “waviness”) of a milled surface is large, typically of the order of millimeters. For this reason, such raster cutting techniques have not traditionally been used in producing optics.
What is highly desirable for the efficient cutting of lenses, particularly customized progressive lens or other aspheric lens designs, is a method of cutting that can produce the required range of aspheric, three-dimension surfaces, and overcomes one or more of the surface quality problems associated with traditional cutting techniques.